The ability to nonverbally enumerate large numbers of items in parallel (e.g., an array of 40 dots) is seemingly contradictory to the limit of 3–4 individual objects that can be stored in parallel in tasks of attention and working memory. However, recent work addressed this paradox by showing that adults can enumerate up to 3 sets of objects (e.g., 10 red, 8 green, and 17 yellow dots) in parallel (Halberda, Sires & Feigenson, 2006). To date, the developmental origin of this ability to enumerate multiple sets remains unclear. By 6 months, infants can enumerate at least 1 set of items (Xu & Spelke, 2000). And by 10 months, they can track up to 3 individual objects in parallel (Feigenson & Carey 2003, 2005). The current studies ask whether the limit on the number of individual objects infants can represent also constrains the number of sets they can represent. Infants were habituated to arrays consisting of 2, 3, or 4 sets of spatially intermixed colored dots (4 – 20 dots per set). They then saw alternating test arrays. In the Discriminable test array, one of the colored sets (randomly determined) changed its numerosity. In the Non-Discriminable test array, all of the sets changed their numerosity by an amount known to be undetectable by infants of the relevant age. Thus, the change in total number of dots was equated across test trial types, yet only in the Discriminable arrays did the number of dots in one of the subsets change substantially. Results show that infants detected a change with arrays containing 2 sets, but not with 3 or 4 sets. Thus, infants, like adults, can track multiple numerosities in parallel. Furthermore, the upper-limit on the number of sets they can enumerate appears to be bounded by constraints on working memory.